Generally, in manufacturing tubes according to the Mannesmann tube making process, a billet (round steel billet) is first pierced by means of a piercer to form a hollow shell, which is then elongated, and rolled in a constant diameter and further subjected to a processing step to produce a seamless tube. The piercer is an inclined rolling mill in which main rolls whose axes are inclined relative to the pass line of the billet to be rolled are used in combination with a plug.
The piercer for developing the Mannesmann tube manufacturing process generally comprises a pair of main rolls that are arranged in an opposing manner with a pass line, a plug for the inner surface regulating tool that is arranged along that pass line, and guide shoes or disk rolls for the shell guide members that are arranged in an opposing manner with a pass line.
FIG. 1 shows an exemplary arrangement of the main rolls in an inclined piercing-rolling mill. FIG. 2 is a cross-sectional view along the line V-V in FIG. 1. FIG. 3 is a view of the inclined piercing-rolling mill as seen from the outlet side.
As shown in FIG. 1, the cone-shaped main rolls 1R, 1L have an outlet side surface angle α and are arranged in axial symmetry with their axes each toe with the pass center m at the toe angle γ.
Further, as shown in FIG. 2, the main roll 1L is arranged to form a feed angle β. The other roll 1R (not shown) is also arranged at a feed angle β, and each of the rolls is in a skew position. The toe angle γ and the feed angle β for these main rolls 1R, 1L are defined relative to the center of the gorge between the main rolls 1R, 1L shown in FIG. 3. The plug 7 is arranged between the main rolls 1, 1 and the tip thereof is positioned in the vicinity of the center of the gorge formed by the main rolls 1, 1.
The main rolls 1R, 1L are respectively connected to the driving force sources 3, 3 via driving shafts 2, 2. By this means, the main rolls 1, 1 are rotated around their respective axes in the direction shown in FIG. 3, for instance. As shown in FIG. 1, the main rolls 1R, 1L are in mutually twisted positions owing to the setting of the feed angle β. When the main rolls 1R, 1L are rotated in the direction indicated by the arrows in the figures, the billet 6 is inserted between the main rolls and pierced while it is rotated in the clockwise direction, as seen from the outlet side, around the pass center m. In this way, the billet is pierced and rolled by the main rolls 1R, 1L and the plug 7 to give a hollow shell 9.
During the piercing in that manner, the billet swings upward and downward due to the pressing forces exerted by the main rolls 1R, 1L. In order to suppress this swinging, a pair of disk plates are arranged above and below the main rolls 1R, 1L.
FIG. 4 shows an exemplary arrangement of the main rolls and disk rolls in an inclined piercing-rolling mill. The disk rolls 10 are arranged symmetrically above and below the main rolls 1R, 1L in the vicinity thereof in a manner sandwiching the billet 6 and are rotated around the respective disk roll shafts 12. These disk rolls 10 are rotated along with the advancement of the billet and suppress the swinging of the billet 6 so that the rolling may proceed smoothly.
However, the main rolls 1R, 1L each has an outlet side surface angle α, as shown in FIG. 1, and further has a toe angle γ to the pass center m, so that there are gaps G, G between the disk rolls 10 and the main rolls 1, as shown in FIG. 4. When the billet 6 rotates and advances along the pass center m, the hollow shell in contact with the surfaces of the disk rolls 10 may be extruded through the gaps G, G.
A movement of the hollow shell formed on the roll outlet side from the gorge between the main rolls is shown in FIG. 5. The bulging of the outside diameter of the billet on the side where it is held by the main roll 1R becomes larger than the bulging of the outside diameter of the billet on the side where it leaves the other main roll 1L. Referring to FIG. 6 in which the disk rolls are also shown, the bulging of the section (section B in the figure), where the billet is held by the main roll 1R, is greater than the bulging in the section (section A in the figure) where the billet leaves the main roll 1L but the bulges are reduced by the pair of the disk rolls 10U, 10D arranged above and below the billet.
In an ordinary piercing-rolling process where the tube expansion ratio, a ratio of the outside diameter of the billet after piercing to the outside diameter of the billet before piercing, [billet outside diameter after piercing]/[billet outside diameter before piercing], is 1.0 to 1.05, therefore the bulging of the billet outside diameter in the section B shown in FIG. 6 causes no problem. However, when piercing-rolling process is carried out at a higher tube expansion ratio, the circumferential length of the hollow shell formed on the main roll outlet side becomes larger, so that the bulging in the section B shown in FIG. 6 increases and the holding angle ø by the main roll 1L also increases. As a result, at the time of billet leakage when the driving force in the rolling direction becomes smaller in an unsteady piercing and rolling state, the billet no longer rotates and buttock clogging occurs, so that the hollow shell end shows a large ellipsoidal shape and causes a problem of formation of shoe mark defects on the outside surface of the hollow shell.
Patent Document 1 below proposes means for solving such problems.
FIG. 7 is a schematic plan view illustrating the arrangement of the main rolls and disk rolls. The main roll 1R is arranged on the right side in the direction of outlet of the billet 6 and the main roll 1L on the left side, with their axes in mutually twisted positions. The main roll 1R is inclined upward on the inlet side and downward on the outlet side, and the main roll 1L is inclined on the opposite manner.
The disk roll 10U is arranged above the billet 6, and the disk roll 10D is arranged symmetrically below the billet 6. The disk roll 10U is arranged so that the outlet side thereof, with the disk roll center as an axis, may be closer to the main roll 1R and parallel to the outlet side surface of the main roll 1R. The disk roll 10D is arranged so that the outlet side thereof may be closer to the main roll 1L and parallel to the outlet side of the main roll 1L. The disk roll 10U forms a disk roll skew angle δ to the pass center m, and the disk roll 10D likewise forms a disk roll skew angle δ.
When the main rolls 1R, 1L are rotated in the direction indicated by the arrows in the figure, the billet 6 is rolled while rotating in the clockwise direction seen from the outlet side. On that occasion, the billet is guided by the disk rolls 10U, 10D so that it may be taken up by the main rolls 1R, 1L at a smaller holding angle ø and thus the billet is prevented from being caught up, according to the description.
Thus, it is proposed that the technology preventing the billet during rolling from protruding by providing an inclination mechanism for varying the skew angle in order to dispose the disk rolls substantially parallel to the outlet surface angle of the main rolls and a moving mechanism for setting the gaps between the main rolls and the disk rolls substantially at zero.
Further, Patent Document 2 below proposes that a pair of cone-shaped main rolls having an inlet surface angle and an outlet surface angle and a pair of disk rolls having a skew angle varying mechanism be arranged alternately around the pass line and a seamless metal tube manufactured by piercing and rolling be carried out at a skew angle such that the skew angle of the disk rolls and the inlet surface angle and outlet surface angle satisfy a specific relation. The buttock clogging or outer surface defects can be prevented thereby even in the case of piercing and rolling at an increased tube expansion ratio of 1.15 or higher.
[Patent Document 1] Japanese Patent Unexamined Publication No. S63-90306
[Patent Document 2] Japanese Patent Unexamined Publication No. H05-124612